Abstract
Bimodal grain structures are common in many alloys, arising from a number of different causes including incomplete recrystallization and abnormal grain growth. These bimodal grain structures have important technological implications, such as the well-known Goss texture which is now a cornerstone for electrical steels. Yet our ability to detect bimodal grain distributions is largely confined to brute force cross-sectional metallography. The present study presents a new method for rapid detection of unusually large grains embedded in a sea of much finer grains. Traditional X-ray diffraction-based grain size measurement techniques such as Scherrer, Williamson–Hall, or Warren–Averbach rely on peak breadth and shape to extract information regarding the average crystallite size. However, these line broadening techniques are not well suited to identify a very small fraction of abnormally large grains. The present method utilizes statistically anomalous intensity spikes in the Bragg peak to identify regions where abnormally large grains are contributing to diffraction. This needle-in-a-haystack technique is demonstrated on a nanocrystalline Ni–Fe alloy which has undergone fatigue-induced abnormal grain growth. In this demonstration, the technique readily identifies a few large grains that occupy <0.00001 % of the interrogation volume. While the technique is demonstrated in the current study on nanocrystalline metal, it would likely apply to any bimodal polycrystal including ultrafine grained and fine microcrystalline materials with sufficiently distinct bimodal grain statistics.
Highlights
Abnormal grain growth (AGG) in polycrystalline materials is defined by a small fraction of grains that grow larger than the vast majority of their neighbors, resulting in a bimodal grain size distribution that does not evolve in a self-similar manner over time
Raw data from a detector scan, such as that shown in Fig. 4, were processed using the Area Diffraction Machine (ADM), an open source code for analyzing area powder diffraction data
Based on the results presented, the use of X-ray synchrotron diffraction to identify abnormal coarse grains in a nanocrystalline matrix shows great potential as a non-destructive and in situ technique
Summary
Abnormal grain growth (AGG) in polycrystalline materials is defined by a small fraction of grains that grow larger than the vast majority of their neighbors, resulting in a bimodal grain size distribution that does not evolve in a self-similar manner over time. The precise mechanism for AGG is a topic of ongoing debate [1], the most common argument centers on a few grain boundaries that have exceptionally high mobility under special conditions [2, 3]. Perhaps the most famous example of a beneficial use of AGG traces back to Goss’ 1935 work on Fe–Si where certain thermomechanical processes were found to produce extreme crystallographic texture (the ‘Goss texture’), enabled by AGG [4]. These alloys, known as electrical steels, have seen extensive industrial use for decades due to their high magnetic anisotropy associated with the strong texture. AGG is problematic in the sintering of undoped alumina ceramics to achieve densities [97 %, an issue that was resolved in the 1960s by the introduction of MgO [5]
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call